Ceria-zirconia Catalysts Research Articles

Correlation between the surface properties and deNOx activity of ceria-zirconia catalysts

The catalytic activities of ceria-zirconia mixed oxides CexZr1−xO2 (x=0.17, 0.42, 0.62 and 0.8) are determined by isothermal steady-state experiments using a representative mixture of exhaust gases of coal combustion. Results show that all supports are active in deNOx reaction in the presence of the previous hydrocarbons. However, their catalytic activity varies with the content of cerium and goes through a maximum for x=0.62, leading to 27% NOx consumption. This activity was correlated with physicochemical properties determined by a linear solvation energy relationship (LSER) approach. Moreover, a mechanism of HC assisted reduction of NO is proposed on ceria-zirconia supported catalysts. This mechanism is divided in three catalytic cycles involving: (i) the oxidation of NO into NO2, (ii) the reaction of NO2 and the hydrocarbons leading to RNOx species and CxHyOz, and finally (iii) the decomposition of NO assisted by these latter CxHyOz species.

Interaction of NO and NO 2 with the surface of Ce xZr 1− xO 2 solid solutions – Influence of the phase composition

Structural (XRD) and spectroscopic (EPR, IR and Raman) investigations were performed to elucidate the influence of CeO 2 content on the phase composition and surface chemistry of Ce x Zr 1− x O 2 solid solutions ( x = 0.10–0.85), interacting with NO and NO 2 in the absence and presence of oxygen. Strong influence of ceria loading on the adsorption modes of both nitrogen oxides and the nature of the resultant surface species was revealed. Adsorption of NO led to formation of mononitrosyl complexes, dimers and N 2O, whereas interaction of NO 2 with the ceria–zirconia catalyst resulted in the adsorbate disproportionation or coupling, depending on the sample composition.

Insights into the dynamics of oxygen storage/release phenomena in model ceria–zirconia catalysts as inferred from transient studies using H 2, CO and soot as reductants

Samples of ceria–zirconia pre-treated under various conditions have been used as catalysts in CO and soot oxidation under stationary and transient conditions, in the presence and in the absence of oxygen. Their behaviour has been compared with that observed under redox conditions in the presence of hydrogen (oxygen storage activity). All the catalysts are active in CO and soot oxidation. Under stationary conditions, the activity in CO oxidation depends on the amount of Ce present, with little contribution from the redox capacity of the support and is strongly influenced by surface area. When the reaction is carried out under transient conditions, especially with low-surface area samples, the performances of ceria–zirconia are higher than those of ceria, with a maximum in the middle composition range. Interestingly, a similar behaviour is observed in soot combustion, where the activity for low-surface area sample is dependent on composition. This suggests that oxygen from the support plays a key role also in the oxidation of large carbon particles under a fully oxidizing mixture.

Effect of preparation methods on the structure and redox behavior of platinum–ceria–zirconia catalysts

Abstract Platinum was introduced into ceria–zirconia solid solutions to form nominal Ce 0.66 Zr 0.32 Pt 0.02 O 2 solid solutions by sol–gel. It was found that, through sol–gel method, the Pt ions could be inserted into the ceria lattice according to the results of XRD, XPS and TPR analyses. This brought about a different synergistic effect between the Pt and Ce components in Ce 0.66 Zr 0.32 Pt 0.02 O 2 compared with the Pt/Ce 0.67 Zr 0.33 O 2 system prepared by traditional impregnation, such as increased the oxygen mobility in ceria–zirconia solid solutions and improved the stability of metallic state for platinum at high temperature. The lower surface Pt content leads to a worse three-way catalytic activity over the Pt-doped sample at low temperature. However, it features a better thermal stability after aging at 950 °C for 20 h, implying a self-generation mechanism of noble metals. This, in return, provides this material with a potential to be used in eliminating pollutants from auto exhaust.

Rietveld refinement and activity of CO oxidation over Pd/Ce0.8Zr0.2O2 catalyst prepared via a surfactant-assisted route

A surfactant-assisted preparation method was successfully used to synthesize the Ce0.8Zr0.2O2 nanophases that had homogeneous chemical composition, enhanced thermal stability and improved catalytic activity. Structural refinement by the Rietveld method confirmed that many cationic lattice defects were formed in the ceria-zirconia crystals due to incorporation of the cationic surfactant in the network of the solid. Zirconium dissolving into cubic structure of ceria improved its thermal stability but led to structure distortion. FT-IR in situ characterization showed that CO chemisorption on the palladium loaded ceria-zirconia catalyst formed linear and multibonded CO–metal complexes. It was also evidenced that CO2 was produced in the CO chemisorption procedure from the pathways of CO disproportionation and CO reacting with lattice oxygen of the catalyst. Owing to high dispersion of the loaded metal palladium and defective structure of the ceria-zirconia support, the Pd/Ce0.8Zr0.2O2 catalyst calcined at a temperature range between 873 and 1173 K not only exhibited a more stable catalytic activity for CO oxidation but also performed a lower light-off temperature at cool start below 373 K in comparison with Pd/CeO2.

Rietveld refinement and activity of CO oxidation over Pd/Ce 0.8Zr 0.2O 2 catalyst prepared via a surfactant-assisted route

A surfactant-assisted preparation method was successfully used to synthesize the Ce 0.8Zr 0.2O 2 nanophases that had homogeneous chemical composition, enhanced thermal stability and improved catalytic activity. Structural refinement by the Rietveld method confirmed that many cationic lattice defects were formed in the ceria-zirconia crystals due to incorporation of the cationic surfactant in the network of the solid. Zirconium dissolving into cubic structure of ceria improved its thermal stability but led to structure distortion. FT-IR in situ characterization showed that CO chemisorption on the palladium loaded ceria-zirconia catalyst formed linear and multibonded CO–metal complexes. It was also evidenced that CO 2 was produced in the CO chemisorption procedure from the pathways of CO disproportionation and CO reacting with lattice oxygen of the catalyst. Owing to high dispersion of the loaded metal palladium and defective structure of the ceria-zirconia support, the Pd/Ce 0.8Zr 0.2O 2 catalyst calcined at a temperature range between 873 and 1173 K not only exhibited a more stable catalytic activity for CO oxidation but also performed a lower light-off temperature at cool start below 373 K in comparison with Pd/CeO 2.

Preparation, characterisation and activity of CeO2-ZrO2 catalysts for alcohol dehydration

Ceria-zirconia catalysts with 100, 75, 50, 25 and 0 mol% of ceria have been prepared and characterised by X-ray diffraction, Raman spectroscopy, N2 physisorption, transmission electron microscopy, X-ray photoelectron spectroscopy and adsorption microcalorimetry of probe molecules. All the catalysts have been tested for the conversion of 4-methylpentan-2-ol into 4-methylpent-1-ene, valuable monomer for plastics manufacture. Steady-state 1-alkene selectivity values ranging between 84 and 59% have been observed. The catalytic behaviour has been interpreted on the basis of competing E1cB, E1 and E2 mechanisms, a well balanced concentration of the acid and basic sites with a high strength of the latter being required for optimum performance.

Surface chemistry and microstructural analysis of Ce xZr 1− xO 2− y model catalyst surfaces

Cerium-zirconium mixed metal oxides are widely used as promoters in automotive emissions control catalyst systems (three-way catalysts). The addition of zirconium in the cubic lattice of ceria improves the redox properties and the thermal stability, thereby increasing the catalyst efficiency and longevity. The surface composition and availability of surface oxygen of model ceria-zirconia catalyst promoters was considered to develop a reference for future catalytic reactivity studies. The microstructure was characterized with X-ray diffraction (XRD) to determine the effect of zirconium substitution on crystalline structure and grain size. Additionally, the Ce/Zr surface atomic ratio and existence of Ce 3+ defect sites were examined with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) for samples with different zirconium concentrations. The surface composition of the model systems with respect to cerium and zirconium concentration is representative of the bulk, indicating no appreciable surface species segregation during model catalyst preparation or exposure to ultrahigh vacuum conditions and analysis techniques. Additionally, the concentration of Ce 3+ defect sites was constant and independent of composition. The quantity of surface oxygen was unaffected by electron bombardment or prolonged exposure to ultrahigh vacuum conditions. Additionally, XRD analysis did not indicate the presence of additional crystalline phases beyond the cubic structure for compositions from 100 to 25 at.% cerium, although additional phases may be present in undetectable quantities. This analysis is an important initial step for determining surface reactions and pathways for the development of efficient and sulfur-tolerant automotive emissions control catalysts.

Study of the Reducibility of Pt or Pd on Ceria-Zirconia Catalysts by XANES measured and the Ce LIII Edge and Magnetic Susceptibility Measurements

The reduction behaviour of Ce x Zr1−x O2 and Pt or Pd supported on Ce x Zr1−x O2 was investigated by means of XANES measured at the Ce LIII edge and magnetic susceptibility measurements. Both techniques agree quite closely for the materials studied. The presence of a noble metal greatly improves the reduction of the Ce x Zr1−x O2 support at low temperatures. The reoxidation that occurred after evacuation of the reduced solids and heating to 500°C suggests that it is linked to a reversible surface reduction process of the sample.

Acid–base properties and catalytic activity of nanophase ceria–zirconia catalysts for 4-methylpentan-2-ol dehydration

The room-temperature high energy ball-milling technique was used to prepare nanophase Ce1-xZrxO2 (x=0; 0.2; 0.5; 0.8; 1) catalysts. The acid–base properties of these catalysts were investigated by means of adsorption microcalorimetry, using NH3 and CO2 as probe molecules. The catalytic activity for 4-methylpentan-2-ol dehydration was tested at atmospheric pressure in a fixed-bed flow microreactor. The inclusion of increasingly high contents of zirconium into the ceria lattice has a complex influence on the acidity and basicity of the pure parent oxide, in terms of both number and strength of the sites. A maximum in 1-alkene selectivity is observed for the ceria-rich catalyst and a minimum for the zirconia-rich sample. Catalytic results are correlated with the acid–base properties and can be interpreted in the light of the mechanism formerly proposed for zirconia, ceria and lanthania. Surface conditioning of the zirconia-rich catalyst occurs during the run, resulting in a remarkable variation of selectivity.

Neutron diffraction studies of ceria-zirconia catalysts prepared by high-energy mechanical milling

The structural transformation occurring to a mixture of cubic CeO 2 and monoclinic ZrO 2 by High-energy Mechanical Milling (MM) has been followed by neutron diffraction. Rietveld analysis indicate that 9 hours of milling produce a tetragonal solid solution for Zr-rich composition and a nanocrystalline cubic solid solution for Ce-rich composition.